1. Field of Invention
This invention relates to a time measuring apparatus which is capable of measuring time interval Tx with higher resolution than the period t.sub.o of a clock signal by measuring start and stop interpolation times. More particularly, this invention relates to a time measuring apparatus which is capable of accurately measuring a very short time interval.
2. Description of the Prior Art
Generally, in order to measure a time interval accurately, the following principle is used. A clock signal having a period t.sub.o is passed to a gate which is opened for a time interval Tx to be measured and the number of clock pulses N which have passed the gate is counted to thereby determine Nt.sub.o as the time interval.
Strictly speaking, according to this method, it does not hold that Tx=Nt.sub.o but that Tx.apprxeq.Nt.sub.o. This is because usually Tx cannot be divided by t.sub.o with a remainder of a small interpolation time. This is shown in FIG. 4, line (c), wherein .DELTA.T.sub.1 denotes the start interpolation time interval from the rising edge of Tx to a clock pulse C.sub.o occurring immediately after the rising edge .DELTA.T.sub.2 (shown at line (d)) denotes a stop interpolation time interval from the falling edge of Tx to a clock pulse C.sub.n occurring immediately thereafter. The gate is opened for the time interval from clock pulse C.sub.o to C.sub.n (see FIG. 4, line (e)) to count clock pulses which pass therethrough. If the number of clock pulses inputted for the time interval is N (see FIG. 4 line (f)), the time interval Tx to be measured is represented by Equation (1) EQU Tx=Nt.sub.o 30 .DELTA.T.sub.1 -.DELTA.T.sub.2 ( 1)
Therefore, it can be understood from Equation (1) that if the interpolation time intervals .DELTA.T.sub.1 and .DELTA.T.sub.2 are measured, the time interval Tx can be measured with a higher resolution than the clock period t.sub.o.
One apparatus for measuring the interpolation time .DELTA.T uses the so-called "time expansion" system which will be described with reference to FIG. 20. In this sytem, for example, a capacitor is charged with a current value of 200.multidot.I for duration of a pulse width .DELTA.T of an interpolation pulse (FIG. 20, line (b)). Thus, the voltage V.sub.p across the capacitor is proportion to .DELTA.T. Thereafter, the capacitor is discharged slowly with a current value, for example, of I. The time (t.sub.3 -t.sub.2) required for discharging the capacitor is proportional to the pulse width .DELTA.T. Thus, an expanded pulse width signal (FIG. 20, line (d)) corresponding to the time interval from the falling edge (time t.sub.1) of the interpolation pulse (FIG. 20, line (b)) to the completion of the discharge (time t.sub.3) is obtained. This expanded pulse width signal T.sub.D is counted using a clock pulse signal (FIG. 20, line (a)) to accurately measure .DELTA.T, namely, the interpolation pulse. Of course, the expanded pulse width signal of FIG. 20, line (d) may be replaced witn a pulse width signal of time interval (t.sub.3 -t.sub.2).
The above time expansion system expands a small pulse width .DELTA.T and counts the expanded pulse width T.sub.D with a clock pulse signal in order to measure the pulse width .DELTA.T accurately without measuring the interpolation time directly. This system requires the expanded pulse width interval T.sub.D, so that the response of the time interval measuring circuit is low. Thus, if the measured time interval Tx (see FIG. 4, line (a)) is inputted repeatedly, the next measurement cannot be performed unless the measurement of the start and stop interpolation pulses is completed. Hence, the frequency of the repeated input time intervals Tx is low. In other words, a short time interval measurement (or put another way, high speed repeated measurement) cannot be conducted.